The present invention relates to a disc brake system, with mechanical operation, for bicycles, particularly suitable for use on mountain, all-terrain and racing bikes, on road or track.
At present, most disc brake systems for bicycles available on the market make use of a hydraulic system to press the brake linings on the disc. All hydraulic disc brakes, according to the known art, have a caliper mounted in a fixed manner and equipped with two opposed pistons which, under the action of the hydraulic system, press the linings against the disc to cause braking. This solution, besides being of excessively complicated construction, requires absolutely precise assembly, to avoid undesirable friction between the linings in the resting position and the disc.
Furthermore, hydraulic disc braking systems require replacement of the normal brake levers attached to the handlebar with other types of levers integrated with the hydraulic circuit.
Heavy use of the brakes causes overheating of the hydraulic system which results in poor operation and the possibility of breakage.
Furthermore, these systems oblige the user to carry out frequent bleeding and/or topping up of the hydraulic circuit, difficult operations even for specialized mechanic-cyclists.
Based on what is described above, hydraulic braking systems are clearly subject to some drawbacks such as an excessive cost, poor resistance to high temperatures, and excessively complex installation and maintenance.
These drawbacks are solved in part by mixed hydraulic and mechanical braking systems. In these known braking systems, the caliper is hydraulically operated, but is actuated by a cable controlled by traditional levers applied on the handlebar. These calipers are mounted, by means of fixing pins, in a floating manner on the fork, so as to allow self-centering during braking. Braking is achieved by means of the pressure of a single small piston connected to the caliper which thrusts a lining against the disc. With this system the friction due to imperfect alignment or a partial return of a lining to the resting position is eliminated.
This mixed braking system, however, has limits to its performance at high temperatures. In fact the "extreme miniaturization" (limited surface of contact during braking) and the difficulty in guaranteeing the complete return of the linings to the resting position under all conditions, prevent heat dissipation. Besides a considerable variability in performance, overheating of the braking system causes boiling of the fluid in the hydraulic circuit and sometimes failure of the seals, which in any case have a limited life.
Because of the above mentioned problems, some attempts at totally mechanical disc brakes have been made in the prior art. These known braking systems use a caliper with a fixed mounting, operated by means of a cable controlled by the connected braking lever on the handlebar. Rolling of two/three balls each on an inclined track is exploited to press the linings against the disc.
This mechanical system, however, also has various drawbacks. The caliper mounted in a fixed manner does not allow self-centering thereof during braking, causing undesirable friction when the linings are in the resting position.
The balls, which have a very small surface of contact, rapidly score the sliding ramps. This causes a considerable increase in friction which, after a few operations, is such as to invalidate the braking system.
These known mechanical brake systems, moreover, cannot achieve the performance in terms of power of the present hydraulic braking systems.